CNTs have excellent electrical characteristics and mechanical strength along with other superior properties, and have been actively researched and developed as an ultimate novel material. CNTs are synthesized by using various methods, including a laser vaporization method, an arc discharge method, and a chemical vapor deposition method (CVD method). However, CNTs produced by using any of the currently available synthesis methods are obtained as a mixture of metallic CNTs and semiconducting CNTs.
Because either one of the metallic and semiconducting properties is often used in actual use, separation and purification of only the metallic or semiconducting CNTs from a CNT mixture is an urgent and important research subject. Further, because different structures (including diameter and chirality; described later) of semiconducting CNTs provide different properties, a technique for obtaining semiconducting CNTs of a uniform structure is strongly needed.
There are reports of separating metallic CNTs and semiconducting CNTs. However, all of these reports pose problems in industrial production of metallic CNTs and semiconducting CNTs, as follows. (1) The separation involves complicated steps, and cannot be automated; (2) the separation is time consuming; (3) mass processing is not possible; (4) expensive equipment and chemicals are required; (5) only one of the metallic CNTs and semiconducting CNTs are obtained; and (6) the collection rate is low.
Examples of the currently available methods include a method in which CNTs dispersed with a surfactant are subjected to dielectrophoresis on microelectrodes (Non-Patent Document 1), a method in which amines are used as the dispersant in a solvent (Non-Patent Documents 2 and 3), and a method for selectively burning semiconducting CNTs with hydrogen peroxide (Non-Patent Document 4). However, these techniques also have the foregoing problems. Particularly, the final material is limited to metallic CNTs, and the collection rate is low.
Other methods includes a method in which a mixture of semiconducting CNTs and metallic CNTs is dispersed in a liquid to selectively bind the metallic CNTs to particles, and in which the metallic CNTs attached to the particles are removed to separate the semiconducting CNTs (Patent Document 1), a method in which CNTs treated with a nitronium ion-containing solution are filtered and heat treated to remove the metallic CNTs contained in the CNTs and to obtain semiconducting CNTs (Patent Document 2), a method using sulfuric acid and nitric acid (Patent Document 3), and a method in which CNTs are selectively separated by migration under applied electric field to obtain semiconducting CNTs confined within a narrow electric conductivity range (Patent Document 4).
These techniques also have the foregoing problems. Particularly, the resulting final material is limited to semiconducting CNTs, and the collection rate is low.
There is also a method in which CNTs dispersed with a surfactant are separated into metallic CNTs and semiconducting CNTs by density-gradient ultracentrifugal separation (Non-Patent Document 5). This technique is also problematic, because the method uses a very expensive ultracentrifugal separator, and requires a long time for the ultracentrifugal separation procedure. Further, because the ultracentrifugal separator can only be increased to a certain size, more than one ultracentrifugal separator needs to be installed in parallel, and accordingly automation and other processes are difficult.
In another method, separation is achieved by ion-exchange chromatography using a CNT-nucleic acid complex of CNTs attached to nucleic acid molecules (Patent Document 5). A problem of this method, however, is that it requires an expensive synthetic DNA, and that the collection rate and purity are poor because of the moderate separation accuracy.
Further, there is a report directed to separating metallic CNTs and semiconducting CNTs under electric field after causing protonation in different extents for different CNTs by adjusting the pH or ion strength of a CNT solution prepared by dispersing the CNTs with a surfactant (Patent Document 6). However, in this method, a pretreatment step needs to be performed before separation with the use of a strong acid for the pH and ion-strength adjustments of a suspended nanotube mixture. The method thus inevitably involves strict step control, and does not successfully separate metallic CNTs and semiconducting CNTs (Patent Document 6, Example 4).
Separation of semiconducting CNTs by diameter or structure also involves problems similar to those identified in the separation of metallic CNTs and semiconducting CNTs.
CNTs can be separated by diameter by density-gradient ultracentrifugal separation (Non-Patent Document 5). However, the technique has problems as above, including the need to use a very expensive device, a long separation time, limited size, and difficulties in implementing automation and other processes.
There is also a report of separating a CNT structure by ion-exchange chromatography using a CNT-nucleic acid complex (Patent Document 7). However, the method is problematic, because it requires a specific synthetic DNA for the CNTs of each individual structure, and that the synthetic DNA is very expensive.
As described above, all of the conventional methods are insufficient for overcoming the foregoing problems, and there is a need for developing a method based on new ideas whereby metallic CNTs and semiconducting CNTs can be separated from CNTs, and whereby semiconducting CNTs of a specific structure can be separated.
The present inventors have worked on a novel method that differs from any of the conventional methods for separating metallic CNTs and semiconducting CNTs, and completed the inventions below (Patent Documents 9, 10, 11, and 12). These inventions enable separation of semiconducting CNTs from metallic CNTs by the selective adsorption of the semiconducting CNTs by a gel used in combination with specific types of dispersant. The semiconducting CNTs adsorbed to the gel are separated from the unadsorbed CNTs by methods such as electrophoresis (Patent Documents 9 and 10), and centrifugation or freeze squeeze, diffusion, and permeation (Patent Document 11). These methods are highly desirable, because both the metallic CNTs and the semiconducting CNTs are obtained, and because the methods enable separation in a short time period at high collection rate, and conveniently enable mass processing with inexpensive equipment.
The present inventors also completed a method that uses a suitable elution for the collection of semiconducting CNTs adsorbed on a gel (Patent Document 12). Specifically, a CNT dispersion is passed through a gel to allow semiconducting CNTs to be adsorbed by the gel. The unadsorbed metallic CNTs are then eluted and separated from the gel, and the semiconducting CNTs adsorbed on the gel are collected with an elution. This technique is very desirable, because the method enables the gel to be used repeatedly, and the separation to be automated for the industrial mass production of metallic and semiconducting CNTs.
In a similar technique invented by the present inventors, the elution concentration is adjusted, and CNTs are separated by diameter simultaneously with the separation of metallic CNTs and semiconducting CNTs (Patent Document 12). This technique is very desirable, because CNTs having different diameters can be obtained simultaneously with the separation of metallic CNTs and semiconducting CNTs, and because the method conveniently enables mass processing and automatic processing in a short time period at high yield with inexpensive equipment.
However, the method suffers from low accuracy in the diameter-specific separation, and requires further improvement for obtaining semiconducting CNTs having a uniform structure.    Patent Document 1: JP-A-2007-31238    Patent Document 2: JP-A-2005-325020    Patent Document 3: JP-A-2005-194180    Patent Document 4: JP-A-2005-104750    Patent Document 5: JP-A-2006-512276    Patent Document 6: JP-A-2005-527455    Patent Document 7: JP-A-2004-142972    Patent Document 8: JP-A-2006-282418    Patent Document 9: JP-A-2008-285386    Patent Document 10: JP-A-2008-285387    Patent Document 11: WO2009/075293    Patent Document 12: Japanese Patent Application No. 2009-147557    Non-Patent Document 1: Advanced Materials 18, (2006) 1468-1470    Non-Patent Document 2: J. Am. Chem. Soc. 127, (2005) 10287-10290    Non-Patent Document 3: J. Am. Chem. Soc. 128, (2006) 12239-12242    Non-Patent Document 4: J. Phys. Chem. B 110, (2006) 25-29    Non-Patent Document 5: Nature Nanotechnology 1, (2006) 60-65    Non-Patent Document 6: Nano Letters 9, (2009) 1497-1500    Non-Patent Document 7: NATURE 460, (2009) 250-253